Service Bulletin: A65 Overheating Carburetor Problem
My Lightning would not hold idle and would in turn have poor throttle response after long periods of riding
I especially noticed this problem during the summer when I would come to a red light or re-starting my motorcycle when at the gas pumps
During my ownership it took some time to isolate and locate what was causing this “overheating” carburetor problem and the symptoms that went along with it
The first telltale sign I found about my overheating carb issue was when I pushed the tickler down, fuel would spray / burst out, rather than trickle down when cold
I ultimately pinpointed the overheating problem due to the fact that the A65 twin carb (IE Lightning / Firebird Scrambler) heads do not have “bolt-on” manifolds which means your carb is bolted directly to the cylinder head - this allows heat to transfer to the carburetor and not allowing the heat to properly dissipate
Speed up to 2021, bike is long gone, however I found this service bulletin recently that covers exactly what I experienced
Let’s cover it below..
Service Bulletin
Click on the image above to enlarge
This 1971 dated bulletin states that the "Overheating Carburetor issue" happens when the surrounding air temp is high and while the engine is running at low RPM's for long periods
To combat this problem, BSA recommends that you must use the 70-2968 insulator block to help prevent heat transfer to your carburetor
BSA does not state this, but the 70-2968 spacers now comes in different thicknesses, bore sizes for different (universal) applications and material (phenolic)
On my A65 I did use the 70-2968 blocks with a thickness of .125 (1/8”), however it did not fix my problem, I ended up going with x2 spacers .125” on each carb with thin carb nuts - that did fix my problem
Not mentioned in the service bulletin, you can also help with lowering the temps of your carburetor and engine by checking your state of tune, spark plug choice and ignition timing within your engine
Purchase
If you are experiencing this problem and need to purchase an insulator block, I recommended clicking on the links below to view our stock
As stated previously, we offer different bore sizes and thicknesses to work with most applications
If you plan on running a thicker insulator block, be mindful of the stud length you have to work with…
| Part type | Part Number | Bore Size | Link |
| Insulator Block | 70-2968 | 26(mm) | Click Here |
| Insulator Block | 70-2968 | 28(mm) | Click Here |
| Insulator Block | 70-2968 | 30(mm) | Click Here |
| Insulator Block | 70-2968 | 32(mm) | Click Here |
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Ride safe!
Thanks for reading
Good Country
With lead being removed from our fuel and all the additives they use to dick around with octane and lowering supplier cost do you think the boiling point of fuel today has been lowered causing some of the problem?
Classic British Spares
@Steve – 70-2968 originally came in 1 bore size. That is why BSA recommends that you open the bore to port match the head and carbs. Common practice even in todays standard… Die grinders / Dremel’s work too. Thanks for reading
Steve
Great article and explains why my 69 lightning cuts out at the lights. Not sure about the suggestion to file asbestos. Presumably when the service bulletin was written we were none the wiser
Peter W
Yep, same as all the rest of the riders here in that my A65 OIF T/bolt did not like temps above 90 F.
But the T/bolts had the removeable inlet manifold with gaskets onto the head.
So that helped insulation but I still needed the two (2) .125" insulation blocks with 3 gaskets.
This worked but pushed the Amal back into the single air filter assy rubber mount and it is a PITA to fit!
My ‘69 B44VS arrived from Tucson with a 30 mm JRC PWK flat slide. Took a number of runs to get the jetting etc right but ’seems’ to run far better on the hotter days. But I still fitted 2 insulators!
Don J
Hi All, We ride no matter how hot. 100f is common. Riding up Central Valley & Sierra foot hills 110f not uncommon.
I’ve done many tests feeling carbs. Even the thin insulator is huge help. Thick insulator is much better. This applies to both BSA & Triumph. I’ve done tests with both thick insulator & air gap with fat oring on Triumphs. It’s a wash which which keeps carb cooler.
100+f moving constantly not much problems. Stop/go traffic with little wind & repeated stop lights in city motor heat is very high. The entire fuel tank & fuel gets very. Let the rpms down at red light can/will stall, & can be hard to start. Worn slide makes this much worse. California fuel is made to always be under some pressure. In a carb you must keep blipping throttle. You’ll feel it when that’s needed as it will want to stall. A higher idle really helps. But who likes too high of idle?
Hot start issues will really scare you. You’d swear motor has no spark.
What my riding buddies & I do is tickle carb like it’s cold. Hold full throttle, kick hard for faster cranking rpm. Be ready to back off throttle, but keep it 3-4000 rpm until carb cools.
Be mindful to keep idle extra high for about 2-3 minutes.
Motor stopped high heat comes up open intake valve & from head in general. No insulator will stop this. On hottest days you stop bike for restroom break. Tickler button is too hot to push with bare finger.
We often leave fuel tap on as evaporating fuel is replaced by cooler tank fuel. Hottest I’ve ridden in is 117f. Either figure out how to deal with the heat or watch TV. I’ll take heat any day.
T140E carbs will do similar hot start problems. Use enrichment lever & hold full throttle. I’ve started many bikes like this.
If either carb then floods. Hold full throttle several kicks. Then kick no throttle then try tickling no throttle. Sometimes they really scare you.
Don